Prior art measuring devices primarily rely upon an electrical bridge concept and there are a variety of modifications of this concept. Bridge measuring systems are widely used and valuable where conditions are fairly predictable and a fine degree of accuracy is desired. Highly accurate measurement systems are not easily or cheaply constructed.
In U.S. Pat. No. 3,418,536, issued to Lee, et al, a different approach to measurements is proposed. In the Lee patent, switching techniques are used to provide pulse width modulated output as a measurement value. To accomplish this, pairs of sensing network channels are provided and cross-triggering for operation of the channels is necessary. The Lee System involves separate reference resistors as well as having error sensitivity in response to temperature and component differences. The system proposed in the Lee patent basically does not solve the problem of eliminating errors due to the components, supply voltage variance and the like.